The present invention relates to a method and a device for the automatic graphic display of I/Q values of a baseband signal by means of a constellation diagram.
A constellation diagram serves for the graphic display of received symbols of a digitally modulated signal. Typically, the signals arriving in a receiver are evaluated for this purpose on the basis of preamble information of the signal, wherein the respective modulation type, the bit-depth per symbol, the signal source and other information can be contained in the preamble of the signal. Alternatively, this information is obtained from especially reserved subcarriers, for example, the Transmission Parameter Signaling, abbreviated as TPS in the Digital Video Broadcast standard DVB-T. As an alternative, the information is simply specified in a fixed manner for each standard, so that a receiver handles the received digital signals according to this fixed specification.
The assigned symbols obtained from the received signal in this manner represent complex numbers in this context. A symbol can thus be displayed as a point in the complex I/Q plane—the constellation diagram. The real part of the symbol is designated as the In-Phase component, abbreviated as I. The imaginary part of the symbol is designated as the Quadrature-Phase component, abbreviated as Q.
For the transmission of information, digital signals are transmitted on a high-frequency carrier by varying the magnitude and the phase of the carrier in such a manner that the carrier adopts one of several specific constellation points in the constellation diagram at every clock transition. Every constellation point codes a given symbol which comprises one or more data bits. In this context, a constellation diagram shows the valid constellations for all permitted symbols. In order to obtain the respective symbol, the precise magnitude and the precise phase of the received signal must be determined for every clock transition.
Dependent upon the modulation method used, a constellation diagram comprises a different number of decision fields. A decision field is an area in the constellation diagram in which one constellation point is disposed in each case. The decision fields are often drawn as a grid in the diagram in order to obtain a visual demarcation of the individual constellation points from one another.
In this context, a signal transmission can be subject to different disturbances. Reference is made here, for example, to the error-vector measurement, the I/Q phase error over time, the I/Q amplitude inequalities, the carrier suppression, the phase jitter, the sinusoidal interference and the deterioration of the signal-noise ratios, wherein this list is not exhaustive.
If disturbances occur in the transmission path, the actual constellation points of a symbol deviate from the associated ideal constellation point of the symbol within a decision field dependent upon the size of the disturbance on the signal. A so-called signal cloud occurs around the ideal constellation points. The smaller the signal cloud of a constellation point is, the less disturbed the received signal is. The larger the signal cloud of a constellation point is, the more disturbances the received signal has been subjected to. Clearly visually organized constellation diagrams are required in order to find and remove these disturbances in the transmission path in investigations of the signal path.
An arrangement with which an I/Q value is displayed for a predetermined registration time as a pixel on a display element is described in DE 195 47 896 A1. For each I/Q value, a frequency of occurrence is determined in order to implement error calculations.
The problem with such an approach is that, with relatively high orders of modulation, very visually disorganized diagrams are obtained. With relatively large disturbances, signal clouds occur which intrude into the decision fields of other constellation points. As a result of this intrusion, different constellation points are confused with one another thereby enormously complicating an error analysis because it is no longer possible to assign an I/Q value to the respective constellation point.
With currently available transmission methods, such as Digital Video Broadcast, especially with the more recent standard DVB-C2 or DVB-T2, it is also problematic that the constellation points of all subcarriers of several symbols in time sequence, also designated as cells, are superposed in the constellation diagram. In the case of signals with a broad bandwidth, these different subcarriers may be disturbed differently, because the transmission channel varies over such a broad frequency range. Furthermore, each of the symbols can, in principle, be modulated differently, so that a differentiation of the symbols in the constellation diagram becomes impossible.
Furthermore, different symbols typically belong to different datastreams, wherein, for an investigation of the signal, only one datastream may optionally be relevant.
Furthermore, different symbols, especially OFDM symbols, within one transmission frame, also designated as a frame, serve different purposes. For instance, a symbol can be a preamble frame, a data frame or an end frame.
In order to implement an accurate error analysis to determine the largest interfering influence, an improved display is necessary.
Consequently there is a need for an approach for the display of constellation diagrams, in which clear visual organization is enhanced, and wherein an increased computational effort should be avoided, and an error analysis should be improved.